In the power and energy industry, transmission systems are less expanded compared to distribution systems because of economic and environmental constraints. As a result, system operators tend to use the full capacity of the transmission lines for power transfer. Consequently, congested transmission lines result in problems such as voltage instability. Voltage instability is a phenomenon which results from increasing power flow in power corridors. Once a region faces its maximum power transfer limit, it can no longer satisfy the demand of its connected loads, and voltage collapse occurs with additional power transfer.
Voltage collapse is characterized by an initial slow progressive decline in the voltage magnitude of the power system buses followed by a final rapid decline in the voltage magnitude. The main symptoms of voltage collapse are low voltage profiles, heavy reactive power flows, inadequate reactive support, and heavily loaded systems. The consequences of voltage collapse often require long system restoration, while large groups of customers remain without power for extended periods of time. This phenomenon can become irreversible if the voltages reach their voltage stability limits, meaning that even if the loads are decreased afterwards, such as through load shedding, the system can no longer reestablish its nominal voltages. Conventional approaches provide an index (SDI—S Difference Indicator) for indicating voltage collapse. However, the SDI approach is only based on local measurements on the load side of a power corridor. Since the delivered power to the load becomes less in the proximity to voltage collapse, the SDI approach loses its sensitivity when needed most.